How to migrate a regular LXC container to a Proxmox LXC container?

Without detailed explanation here is how you can do this.

1. Create an archive of your LXC container.
2. Create a Proxmox Container using that archive as a template.

First cd into your lxc container root directory:

cd /var/lib/lxc/debian8/rootfs/

(If you used an lvm volume as your containers storage, you need to mount it and cd into your mount point, which can be done by mount /dev/mapper/<lvgroup-lvdisk> /var/lib/lxc/debian8/rootfs/, and than cd into the mount point)

Create an archive:

tar -czvf /var/lib/vz/template/cache/my_debian8_template.tar.gz ./

Then create a new proxmox container out of that

pct create 100 /var/lib/vz/template/cache/my_debian8_template.tar.gz \
    -description LXC -hostname pvecontainer01 -memory 1024 -nameserver 8.8.8.8 \
    -net0 name=eth0,hwaddr=52:4A:5E:26:58:D8,ip=192.168.15.147/24,gw=192.168.15.1,bridge=vmbr0 \
    -storage local -password changeme

(You can modify your options as you would like.)

For more information see - man pct